The Lag and K routing method is a hydrologic storage routing method based on a graphical routing technique that is extensively used by the National Weather Service (NWS) (Linsley, Kohler, & Paulhus, 1982). Within this method, lag and K represent translation and attenuation, respectively. The method is a special case of the Muskingum method where channel storage is represented by the prism component alone with no wedge storage (i.e. Muskingum X = 0). The following equation is combined with inflow vs. translation and outflow vs. attenuation functions in order to solve for outflow:
1)
\frac{d S}{d t}=I_t-O_t
where dS/dt = time rate of change of water in storage at time t; It = average inflow to storage at time t; and Ot = outflow from storage at time t.
The lack of wedge storage means that the method should only be used for slowly varying flood waves. Also, this method does not account for complex flow conditions such as backwater effects and/or hydraulic structures.
Required Parameters
The parameters that are required to utilize this method within HEC-HMS are the initial condition, a lag method and value or function, and a K method and value or function. Two options for specifying the initial condition are included: outflow equals inflow and specified discharge [ft3/sec or m3/sec]. The first option assumes that the initial outflow is the same as the initial inflow to the reach from the upstream elements which is equivalent to the assumption of a steady-state initial condition. The second option is most appropriate when there is observed streamflow data at the end of the reach. Two options for specifying a lag method are included: Constant Lag [hours] and Variable Lag. When using the Variable Lag option, an Inflow-Lag function (which is a paired data object) must be specified. Similarly, two options for specifying a K method are included: Constant K [hours] and Variable K. When using the Variable K option, an Outflow-Attenuation function (which is a paired data object) must be specified. These relationships are typically derived through evaluation of historical flood hydrographs. Care must be exercised when using lag functions with multiple intercepts (i.e., lag is the same for more than one flow rate) as this may result in numerically attenuated peak flow rates.
